This invention relates to ionically cross-linked siloxane polymers. More particularly, this invention relates to ionically cross-linked zwitterionic siloxane polymers having ionic cross-links at trifunctional silicone atoms and a method of their production.
Zwitterions are ions which are both positively and negatively charged. Common zwitterionic species are the amino sulfonates, NH.sub.2.sup.+ --R--SO.sub.3.sup.- and the amino carbonates, NH.sub.2.sup.+ --R--COO.sup.- ; wherein R is a divalent hydrocarbon radical more particularly defined below. Zwitterionic species are typically obtained from ionizing amino acids and the like; however, siloxane polymers containing zwitterions have been prepared by Litt and Matsuda, J. Polymer Science, Vol. 19, p. 1221 (1975) and by Graiver et al., J. Poly. Sci., Polymer Chem. Ed., Vol. 17, p. 3559 (1975). The contents of these articles are incorporated herein by reference.
Litt and Matsuda disclose a process for producing zwitterionic silanes by reacting the trifunctional aminoalkyl silanes, .gamma.-aminopropyltriethoxysilane and N-aminoethyl-.gamma.-amino-propyltrimethoxy silane, with .gamma.-propane sultone.
Graiver et al. disclose that siloxane polymers containing zwitterions can be obtained by treating an aminoalkyl siloxane with .gamma.-propane sultone. The aminoalkyl siloxanes are provided by copolymerizing a dimethoxy silane having an aminoalkyl radical with a low molecular weight hydroxy-terminated polydimethylsiloxane and decamethyltetrasiloxane.
The zwitterions on the siloxane polymers provide ionic cross-linking between the siloxane polymers due to the coulombic forces exerted by the ions. An example of an ionic cross-link which may exist between two siloxane polymer segments is illustrated in the following formula: ##STR1## wherein R' is a divalent hydrocarbon radical of from 1 to 20 carbon atoms and R is a divalent hydrocarbon radical of from 2 to 20 carbon atoms.
These cross-links reduce the mobility of the polymer segments and increases their stiffness. For example, polydimethylsiloxanes (DP=500) are typically liquid at room temperature, yet corresponding zwitterionic polysiloxanes are solid rubbers at this temperature. Introducing zwitterions to as few as 0.5% of the silicone atoms within a siloxane fluid will provide a solid elastomeric material.
These elastomeric materials exhibit high adhesion to glass and other substrates such as, for example, wood, metal, polycarbonates, polystyrene, polyphenylene oxides and blends thereof, etc. The elastomeric properties and adhesive properties of these zwitterionic siloxanes make them suitable for use as adhesives, elastomeric adhesives, sealants, coatings, injection moldable and compression moldable rubbers and plastics, and various silicone based rubbers.
In the present state of the art, only difunctional silanes are utilized to obtain zwitterionic siloxane polymers having a degree of polymerization sufficiently high to provide the useful elastomeric materials described above. Difunctional zwitterionic siloxanes are either copolymerized with dimethyl siloxane oligomers or difunctional aminoalkyl silanes are copolymerized with dimethyl siloxane oligomers and subsequently reacted with .gamma.-propanesultone to obtain the zwitterionic species on the siloxane polymers. It is difficult to prepare the difunctional zwitterionic silanes and the difunctional aminoalkyl silanes, which makes the production of zwitterionic siloxane polymers expensive. It is desirable to utilize less costly precursors in the production of zwitterionic siloxane polymers.
Trifunctional aminoalkyl silanes are more readily available and less expensive than their difunctional counterparts. However, copolymerization of such trifunctional aminoalkyl silanes with dimethyl siloxane by conventional methods has been difficult, if not impossible, to achieve. Typically the trifunctional aminoalkyl silane polymerizes with itself to form a yellow precipitate and does not become incorporated within the siloxane polymer. The present invention is based on the discovery of an effective method for copolymerizing the less expensive trifunctional aminoalkyl silanes or trifunctional zwitterionic silanes with siloxane oligomers to provide aminoalkyl siloxane polymer intermediates and zwitterionic siloxane polymers, respectively. Only a small quantity of the trifunctional aminoalkyl silanes and trifunctional zwitterionic silanes homo-polymerize in this process, which permits a greater proportion to be incorporated within the copolymer produced.